Typically certain motors require a sinusoidal current to be applied for smooth torque production. However, the power electronics that are used to generate the sinusoidal currents do not function well when the current magnitude is small.
A controller, such as a microcontroller containing a central processor, memory, etc., is commonly used to control brushless DC motors. The controller controls a pulse width modulator (PWM) which in turn controls the speed and operation of the motor by modulating the pulse width signals which drive power inverters.
The inverter portion typically includes three half-bridge sections, each corresponding to a respective motor power phase. Rectifiers are also provided to prevent damage to the transistors during dead time intervals in which an inverter is driving neither high nor low.
In operation, the PWM provides output signals whose duty cycle at particular points in time is proportional to the magnitude of the voltage applied to the motor. When changing between the pull up transistor and the pull down transistor, the transistors are changed between conductive and non-conductive states, and there is a short period of time during which neither transistor is conductive. This period, known as the “dead time interval”, and whose effect is shown by the non-sinusoidal lines at the zero crossing of two phase prior art motor currents in FIG. 1, is necessary to avoid the pull up and pull down transistors both being conductive at the same time which could cause damaging shoot-through currents.
As this “dead time interval”, distortion effect occurs each time the sinusoidal current crosses zero, a nonlinear distortion, shown in FIG. 1, appears in the sinusoidal current that disrupts the sinusoidal nature of the applied current and causes non-smooth torque to be produced.
Various techniques had been proposed for correcting this distortion error, such as by using an off-set in the PWM output signal as a function of the out-put current polarity, a current sensing technique, and a current sensing technique which compensates for signal distortion using analog techniques.
U.S. Pat. No. 5,764,024 discloses another “dead time distortion” correction circuit. However, this circuit used hardware voltage sensing thereby requiring several additional components in the circuit to detect a zero-crossing event, which add to the cost of the motor controller.
Thus, it would be desirable to provide a zero-crossing correction method and apparatus for use in sinusoidally commutated motors which can be implemented at a lower cost than previously devised zero-crossing correction techniques.